This was originally intended to but used as the model for the File List View ( which is accessible by running marble with --enableFileView ) but has since been replaced by another, more centralised, File Model. This new File Model is located in PlacemarkManager.
The model that is in the GpsLayer is currently only being used as a collection class and is deprecated. This, along with the whole current GPX implementation, needs to be removed and updated to the current concepts in Marble. See Gpx Status